1. Field of the Invention
This invention relates to a sealing system for acoustic wave touchscreens, for protecting components outside of the touch-sensitive area from contaminants.
2. Description of Related Art
An acoustic touchscreen has a touch-sensitive area on which the occurrence and location of a touch is sensed via the touch's effect on the transmission of acoustic waves thereacross. A common type of acoustic touchscreen employs Rayleigh waves (a term which, as used herein, subsumes quasi-Rayleigh waves). Illustrative disclosures relating to Rayleigh wave touchscreens include Adler, U.S. Pat. No. 4,642,423 (1987); U.S. Pat. No. 4,645,870 (1987); U.S. Pat. No 4,700,176 (1987); U.S. Pat. No. 4,746,914 (1988); U.S. Pat. No. 4,791,416 (1988); and Re 33,151 (1990); Adler et al., U.S. Pat. No. 4,825,212 (1989); U.S. Pat. No. 4,859,996 (1989); and U.S. Pat. No. 4,880,665 (1989); Brenner et al., U.S. Pat. No. 4,644,100 (1987); Davis-Cannon et al., U.S. Pat. No. 5,739,479 (1998); and Kent, U.S. Pat. No. 5,708,461 (1998) and U.S. Pat. No. 5,854,450 (1998). Acoustic touchscreens employing other types of acoustic waves such as Lamb or shear waves, or combinations of different types acoustic waves (including combinations involving Rayleigh waves) are also known, illustrative disclosures including Kent, U.S. Pat. No. 5,591,945 (1997) and U.S. Pat. No. 5,854,450 (1998); Knowles, U.S. Pat. No. 5,072,427 (1991); U.S. Pat. No. 5,162,618 (1992); U.S. Pat. No. 5,177,327 (1993); U.S. Pat. No. 5,243,148 (1993); U.S. Pat. No. 5,329,070 (1994); and U.S. Pat. No. 5,573,077; and Knowles et al., U.S. Pat. No. 5,260,521 (1993). The documents cited in this paragraph are incorporated herein by reference.
FIG. 1 illustrates the operation of a typical acoustic touchscreen 1, having an active, or touch-sensitive area 2. A first transmitting transducer 3a is positioned outside of touch-sensitive area 2, acoustically coupled to the surface of touchscreen 1, and sends an acoustic signal in the form of an acoustic wave 11a traveling parallel to the top edge of touchscreen 1 and generally in the plane of touchscreen 1. Aligned in the transmission path of acoustic wave 11a is a linear array of partially acoustically reflective elements 4a, each of which partially reflects (by approximately 90.degree.) and partially transmits the acoustic signals, creating a plurality of acoustic waves (exemplarily 5a, 5b, and 5c) traveling vertically (parallel to the Y-axis) across touch-sensitive area 2. (The spacing of reflective elements 4a is variable to compensate for the attenuation of the acoustic signals with increasing distance from first transmitter 3a ) Acoustic waves 5a, 5b, and 5c, upon reaching the lower edge of touchscreen 1, are again reflected by approximately 90.degree. (arrow 11b) by another linear array of similarly partially acoustically reflective elements 4b towards a first receiving transducer 6a, where they are detected and converted to electrical signals for data processing. Along the left and right edges of touchscreen 1 are located a similar arrangement. A second transmitting transducer 3b generates an acoustic wave 12a along the left edge, and a linear array of partially acoustically reflective elements 4c creates therefrom a plurality of acoustic waves (exemplarily 7a, 7b, and 7c) traveling horizontally parallel to the X-axis) across touch-sensitive area 2. Acoustic waves 7a, 7b, and 7c are redirected (arrow 12b) by yet another linear array of partially acoustically reflective elements 4d towards receiving transducer 6b, where they are also detected and converted to electrical signals.
If touch-sensitive area 2 is touched at position 8 by an object such as a finger or a stylus, some of the energy of the acoustic waves 5b and 7a is absorbed by the touching object. The resulting attenuation is detected by receiving transducers 6a and 6b as a perturbation in the acoustic signal. Analysis of the data with the aid of a microprocessor (not shown) allows determination of the coordinates of position 8.
Touchscreen 1 may be either a separate plate (typically made of glass, but other hard substrates may be used) mounted over a display panel such as a cathode ray tube (CRT), a liquid crystal display (LCD), plasma, electroluminescent, or other type of display. Alternatively touchscreen 1 may be constructed directly on the face of the display panel (e.g., CRT or plasma) itself
In normal usage a housing 9 (outline indicated by a dotted line in FIG. 1), typically made of molded polymer, is associated with touchscreen 1. Housing 9 contains a bezel 10 (outline also indicated by a dotted line in FIG. 1) that overlays touchscreen 1, concealing the transmitting and receiving transducers, the reflective elements, and other components, but exposing touch-sensitive area 2. This arrangement protects the concealed components from contamination and/or damage, presents a more aesthetically pleasing appearance, and defines the touch-sensitive area for the user.
Bezel 10 may be spaced apart from touchscreen 1. In an abutted configuration bezel 10 attenuates the acoustic signals, reducing the touchscreen's responsiveness. However, a spaced-apart bezel 10 leaves a gap through which contaminants (such as dirt, dust, and, especially, liquids) may enter and damage or interfere with the function of the concealed components. Acoustic touchscreens intended for operation in outdoor environments or in facilities such as restaurants or factories, where exposure to rain, fog, spills, sprays, or cleaning solutions is a likelihood, are especially vulnerable in this regard.
Theoretically, one can form a liquid-impermeable seal by applying a caulking between bezel 10 and touchscreen 1 around the perimeter of active area 2, but the caulking will absorb acoustic energy, interfering with touchscreen operation. Rayleigh wave touchscreens, because of the surface-propagating nature of their acoustic waves, are especially likely to be adversely affected. Borucki, U.S. Pat. No. 5,332,238 (1994) (hereinafter the "Borucki patent," incorporated by reference) states that a caulking will not only absorb significant amounts of acoustic energy so as to render the touchscreen inoperable, but also acoustically couple the screen and can cause a false touch to be registered around the entire perimeter of the screen.
The Borucki patent's solution to the sealing problems to employ a foam strip compressed between the bezel/housing and the touchscreen. Acoustic attenuation is limited to an acceptable level by placing an open-cell surface of the foam against the touchscreen or by restricting contact with the touchscreen to a comer of the foam.
An alternative sealing arrangement is disclosed in Armstrong et al., U.S. Pat. No. 5,784,054 (1998) ("Armstrong patent," incorporated herein by reference), in which a sealing strip made of closed cell foam (or, alternatively, expanded polytetrafluoroethylene) is preferably adhesively affixed to the bezel.
Whatever the sealing system, it is operationally desirable to limit acoustic signal loss attributable to the sealant to less than -6 dB.
A disadvantage of the aforementioned foam sealants is that they rely entirely on compressive forces to hold the sealant in contact with the touchscreen surface. In the case of vibration or loosening of the brackets holding the housing and the touchscreen together, there may be a momentary or permanent increase in the gap between the bezel and the touchscreen, compromising the integrity of the seal. Another limitation of foams is that they may permit the passage of liquids into the protected areas of the touchscreen by a wicking or capillary action. Yet another limitation of such seals is that in order to be effective, the foam strip must be relatively wide, on the order of about 4 mm. This extra width reduces the size of the active area of a touchscreen in comparison to the bezel-covered, or inactive areas of the touchscreen. Also, for a given sealant composition, increased width means increased acoustic absorption, making a narrower seal design more desirable. Thus, it is desirable to develop a sealing system for acoustic touchscreens overcoming one or more of the aforementioned limitations.